...or I could multiclass at 5th level, concentrating on becoming a fighter and gaining a fighter bonus feat (Power Attack?), and again at 6th level (gaining a second fighter feat -- unfortunately Leap Attack is not a fighter feat), and a bonus feat for being 6th level (Leap of Heavens).

...but Catfolk Pounce (Races of the Wild, page 148) is a fighter feat! This feat allows catfolk to charge and make a full attacks (multiple blows) against flat-footed opponents. I like Leap Attack, but this is good, too.

Perhaps instead of Power Attack, I could take Improved Initiative (PHB 96, granting +4 on initiative rolls) as a 5th level fighter; this would help Mirri with charging and attacking flat-footed opponents.